Polyphosphazenes containing repeating phosphonitrilic units which have various alkoxy, substituted alkoxy, aryloxy and substituted aryloxy groups attached to the backbone phosphorous atoms and their method of preparation are described extensively in the prior art. For example, see "Nitrogen-Phosphorous Compounds", Academic Press, New York, N.Y., 1972 by H. R. Allcock, and also "Poly(organophosphazenes)", Chemtech, Sept. 19, 1975 by H. R. Allcock as well as U.S. Pat. Nos. 3,700,629; 3,702,833 and 3,856,712. The disclosures of these patents are incorporated herein by reference.
U.S. Pat. No. 3,700,629 discloses high molecular weight polyphosphazene copolymers having increased thermal stability. These copolymers contain randomly distributed substituents on the phosphonitrilic backbone that are fluoroalkoxy and substituted aryloxy, the substituents including C.sub.1 to C.sub.5 alkyl. The ratio of such fluoroalkoxy groups to aryloxy groups covers a range of 1:9 to 3:1.
While curable fluoroalkoxy and/or aryloxy elastomeric polyphosphazene copolymers are known, these compositions utilize groups displaying some degree of olefinic unsaturation to effect the cross-linking (curing). See for example U.S. Pat. Nos. 3,702,833 and 4,116,785.
The present invention is directed to the preparation of cured (cross-linked) polyphosphazenes using, as a cross-linking site, a chemical group that has heretofore not been known to take part in the cross-linking reaction.
Briefly, cross-linked polymers having enhanced elongation properties are prepared using conventional cross-linking agents, such as peroxides (benzoyl peroxide, dicumylperoxide, etc.) or radiation, with the phosphonitrilic polymers having the units: ##STR2## where the total number of units ranges from about 3 to about 50,000, R.sub.1 and R.sub.2 are different and are C.sub.1 to C.sub.12 linear or branched alkyl, C.sub.2 to C.sub.15 linear or branched fluoroalkyl or mixtures thereof, and R.sub.3 is phenyl substituted with C.sub.1 to C.sub.12 linear or branched alkyl.
In the preferred compositions of the present invention, R.sub.1 and R.sub.2 are C.sub.1 to C.sub.6 linear or branched alkyl, C.sub.2 to C.sub.15 linear or branched fluoroalkyl, or mixtures thereof. In such preferred composition R.sub.3 is preferably phenyl substituted with C.sub.1 to C.sub.6 linear or branched alkyl.
Most preferred compositions of the present invention include those where R.sub.1 and R.sub.2 are HCF.sub.2 (CF.sub.2).sub.n CH.sub.2 -- or F.sub.3 C(CF.sub.2).sub.n CH.sub.2 -- where n is an integer of from 0 to 13. The value of n need not be the same on each fluoroalkoxy substituent, because mixtures of fluoroalkoxy groups are included within the scope of this invention. Examples of most preferred alkyl substituents in R.sub.3 include methyl, ethyl, n-propyl, isopropyl and the like. Typical examples of the above mentioned fluoroalkyl groups include 2,2,2-trifluoroethyl, 2,2,3,3,3-pentafluoropropyl, 2,2,3,3-tetrafluoropropyl, 2,2,3,3,4,4,4-heptafluorobutyl, 2,2,3,3,4,4,5,5- octafluoropentyl and the like.
Among the most preferred polymers of the present invention, particularly preferred are those where R.sub.1 is --CH.sub.2 CF.sub.3 and R.sub.2 is HCF.sub.2 (CF.sub.2).sub.n CH.sub.2 -- where n has a value of 1 to 13, averaging about 3.
The groups --OR.sub.3 are effective in the cross-linking reaction at concentrations from about 0.1 to about 20 mole % based on total moles of substituents in the polymer. The cross-linking agents are typically brought in contact with the uncured polymer by milling or otherwise mixing the polymer so as to uniformly disperse the additives. Curing is typically accomplished by heating the milled compositions to temperatures ranging from about 100.degree. C. to about 200.degree. C.
The overall result is to produce a cross-linked polymer of enhanced elongation useful as a sealing material and for gaskets, adhesives, coatings and the like.
The polymers of the present invention are prepared by reacting at least three different alkali metal salts with linear soluble [PNCl.sub.2 ].sub.x polymer. For example, a benzene solution of the linear, soluble [PNCl.sub.2 ].sub.x polymer is combined with one or more (if reacted step-wise) solutions of the sodium salts of the alcohols R.sub.1 OH, R.sub.2 OH and R.sub.3 OH in tetrahydrofuran and reacted under suitable conditions. A polymer containing random units of the structures set forth above results. These polymers are high molecular weight materials and are completely substituted. They typically have low glass transition temperatures and are usually thermally stable to over 300.degree. C. as measured on a thermobalance.
The solubilities of these polymers vary widely with the nature of the substituent groups. Some materials are soluble in most common organic solvents, while other polymers are soluble only in certain fluorocarbon solvents.